Mechanistic Study On The Treatment Of Acid Mine Drainage By Bio-enhanced Subsurface Flow Constructed Wetlands With Different Substrates

Acid mine drainage（AMD）has become a major problem worldwide,and is characterized by high acidity,heavy metal and sulfate concentrations,which not only pollute surface and groundwater resources,but also kill aquatic organisms and crops,and threaten human health through the food chain.Studies have shown that AMD can continue to be produced for hundreds of years even after mining activities have ceased,so there is an urgent need to propose economical and efficient treatment technologies to minimize its negative effects.In this paper,we conducted an experimental study on the treatment of AMD by submerged constructed wetlands with loess-steel slag substrate under bioreinforcement,and the results of the study can provide a theoretical basis for the prevention and control of AMD,which is important for the protection of water resources and ecological environment.In this paper,the dominant plants suitable for treating AMD were selected through hydroponic experiments;sulfate-reducing bacteria（SRB）were domesticated with corn straw fermentation broth,and the optimal growth conditions of SRB were determined through static experiments;the removal effect of pollutants in AMD under the optimal growth conditions was investigated;a submerged constructed wetland system was constructed to investigate the removal effect and mechanism of pollutants in AMD under two conditions:no additional carbon source in the feed water of the wetland and the addition of corn straw fermentation broth as the carbon source.Microbial communities in two substrate constructed wetland systems in two experimental phases were analyzed using high-throughput sequencing techniques.The following main conclusions were obtained.（1）The growth of six wetland plants（Phragmites australis,Typha orientalis,Cyperus glomeratus,Scirpus validus,Iris wilsonii,Juncus effusus）under AMD stress was investigated through hydroponic experiments,and the removal effects and removal mechanisms of the six plants on pollutants in the AMD were analyzed,and finally Iris wilsonii was selected as the dominant plant for treating AMD and used to construct the constructed wetland system in subsequent experiments.（2）The most suitable solution initial p H range for SRB growth and metabolism is 6-9,the best initial SO₄^2-concentration range is 500-1500 mg/L,and the best COD/SO₄^2-ratio is 2.5.SRB removed 99.45%of SO₄^2-,more than 99%of Fe,Zn,Cu,Cd,and 42.20%of Mn in 11 days under optimal growth and development conditions,indicating that corn straw fermentation broth can be efficiently utilized by the domesticated SRB to achieve good treatment effect on AMD.（3）AMD was treated by constructing submerged wetland systems with loess and steel slag substrates.In the first phase of the experiment（wetland influent with only AMD and no additional carbon source）,the results of monitoring the redox potential,dissolved oxygen concentration and p H of the wetland systems showed that both wetland systems were not suitable for the growth and development of SRB in this phase.In the later stage of this phase of the experiment,the removal rates of most metals were on a decreasing trend（Cu was on an increasing trend）,and at 120 days of the experiment（at the end of the first phase of the experiment）,the removal rates of Fe,Mn,Zn,Cu,and Cd by the loess wetland system were about 97%,8.5%,22%,98.8%,and 0%,respectively,and the steel slag wetland system for Fe,Mn,Zn,Cu,and Cd removal rates were about 97.6%,63%,79%,97.8%,77%,and the wetland systems of the two substrates had basically lost the removal effect of SO₄^2-at this time,indicating that the constructed wetland systems were less effective in the treatment of AMD when the microbial effect is small or even absent.（4）In the second phase of the experiment（AMD and corn stover fermentation broth were mixed as wetland influent）,the redox potential,dissolved oxygen concentration and p H in both wetland systems met the growth requirements of SRB,indicating that the addition of corn stover fermentation solution created a more suitable environment for the growth of SRB in each wetland system,thus improving the removal rate of SO₄^2-and each metal in each wetland system in the second phase of the experiment.The removal rates of Fe,Mn,Zn,Cu,and Cd by the loess wetland system at the end of this phase of the experiment were increased to about 99.9%,30%,95%,99.9%,and 99%,respectively,and the removal rates of Fe,Mn,Zn,Cu,and Cd by the steel slag wetland system were increased to about 99.9%,78%,95%,99.8%,and 98%,respectively.Except for Mn,the effluent concentrations of all metal ions meet the primary standard of GB8978-1996《Integrated Wastewater Discharge Standard》.And due to the better growth and development of SRB in the loess substrate wetland system,the removal rate of SO₄^2-by the steel slag substrate wetland system（eventually stabilized at about 25%）was lower than that of the loess substrate wetland system（eventually stabilized at about 50%）at this stage,indicating that the development of SRB had a significant effect on the treatment capacity of the wetland system.（5）The results of morphological analysis of each metal showed that the mobility of Fe was low in both wetland matrices.Mn was mainly removed by the formation of Mn CO₃ precipitation with carbonate minerals,and the potential hazard of Mn was high.Cd was mostly present in the exchangeable state,indicating that Cd was highly mobile,highly bioavailable and potentially hazardous.Zn was mostly present in the Fe-Mn oxide bound and carbonate bound states.The presence of Cu in the organic state in the second phase of the experiment indicates that the removal of Cu in this phase was mainly dependent on the action of SRB.In general,the percentage of organic state of each metal increased to different degrees in the second phase of the experiment,indicating that SRB contributed to the removal of each metal.（6）The bioaccumulation factor of plants for each metal in both substrate wetland systems were greater than 1,indicating that plants in both substrate wetland systems could extract metals from AMD,and the bioaccumulation factor of plants for each metal in the loess substrate wetland system were greater than those in the steel slag substrate wetland system due to the better growth of plants in the loess substrate wetland system.The transfer factor of plants to each metal in both wetland systems were less than 1,indicating that plants would accumulate harmful substances in the roots,thus reducing the threat to plant growth caused by heavy metals and other harmful substances and maintaining the normal life activities of the plant body.（7）The results of microbial high-throughput sequencing showed that the number of species and community diversity and uniformity of microbial communities in the wetland systems of the two substrates in the second phase of the experiment increased substantially due to the addition of corn straw fermentation broth,which provided sufficient nutrients to various microorganisms in the systems.The relative abundance of SRB in both substrate wetland systems increased,and the relative abundance of SRB in the loess and steel slag substrate wetland systems reached 8.86%and 5.84%,respectively,with the higher relative abundance of Desulfofarcimen and Desulfovibrio.